1. Field of the Invention
The present invention relates to an image pickup apparatus having normal- and high-resolution modes and having the capability of auto focusing in both normal- and high-resolution modes.
2. Description of the Related Art
In conventional cameras for taking an image of an object via an image-sensing device, pixels are disposed at discrete positions on the image-sensing device. As a result, in theory, foldover distortion occurs when a subject having high spatial frequencies is taken. To avoid the above problem, it is known in the art that a crystal low pass filter (hereafter referred to as a crystal LPF) is provided in an imaging optical path so that the birefringence of the crystal LPF causes a proper reduction in the sharpness of the image of the subject thereby reducing the high frequency components contained in the image of the subject to a level lower than the Nyquist frequency determined by the pixel pitch of the image-sensing device and thus preventing the foldover distortion. In particular, in an image pickup apparatus for taking a color image, each pixel of an image-sensing device is covered with a color filter of R (red), G (green), or B (blue) so that luminance and color information is produced on the basis of information obtained from a plurality of pixels. In this structure, color moire can occur if the high frequency components greater than the Nyquist frequency determined by the arrangement of the color filters are present in the image of the subject. The crystal LPF is effective to prevent the color moire. The Nyquist frequency determined by the color filter pattern disposed over a plurality of pixels is lower than that determined by the pixel pitch. This means that the crystal LPF for use in taking a color image causes a greater reduction in the resolution of the image.
Therefore, when it is desired to take a high-resolution image of a subject basically consisting of black and white such as a document in which a certain degree of color moire or foldover distortion will be allowed, if a color image pickup apparatus having a color crystal LPF is employed to take such an image, then high resolution cannot be achieved although the image-sensing device is inherently capable of providing high resolution. One known technique to solve the above problem is to provide an additional crystal LPF that can pass higher spatial frequencies than a normal crystal LPF so that when a high resolution is desired the crystal LPF for use in a normal resolution mode is replaced by the additional crystal LPF.
In the above-described arrangement, however, when an image is taken in the high-resolution mode, the allowable focusing error in the automatic focusing adjustment of the imaging lens system also becomes smaller. As a result, in the high-resolution mode, the depth of focus also becomes smaller, and it is required that the automatic focusing mechanism of the imaging lens system should be controlled with a higher positioning accuracy. In the automatic focusing adjustment technique in which the degree of focusing is evaluated on the basis of the amount of high frequency components contained in a video signal, it is also required that the rate of sampling the evaluation value should be adequately set so that high enough accuracy be obtained in the automatic focusing adjustment. In the conventional techniques, such the considerations on the automatic focusing adjustment in particular in the high-resolution mode are not taken into account.
It is an object of the present invention to solve the above problems. More specifically, it is an object of the invention to provide a camera having both normal- and high-resolution modes, capable of performing automatic focusing adjustment in an optimum manner corresponding to the selected mode.
According to an aspect of the invention, to achieve the above objects, there is provided an image pickup apparatus including an imaging system including an imaging lens and an image-sensing device for converting an optical image formed via the imaging lens into an electric signal, the image pickup apparatus having the capability of performing focusing operation by driving a part of the imaging system, the image pickup apparatus having a normal-resolution image-taking operation mode in which an image is taken with a normal resolution and also having a high-resolution image-taking operation mode in which an image is taken with a higher resolution than the normal resolution, the image pickup apparatus being characterized in that in the focusing operation the minimum amount of movement in driving the part of the imaging system is set to a value which differs between the normal-resolution image-taking operation mode and the high-resolution image-taking operation mode.
According to another aspect of the invention, the minimum driving amount employed in a later stage of the focusing operation is set to a value which differs between the normal-resolution image-taking operation mode and the high-resolution image-taking operation mode.
According to still another aspect of the invention, there is provided an image pickup apparatus including an imaging system including an imaging lens and an image-sensing device for converting an optical image formed via the imaging lens into an electric signal, the image pickup apparatus having the capability of performing focusing operation by driving a part of the imaging system on the basis of the change in the focus evaluation value extracted from the output of the image-sensing device, the image pickup apparatus having a normal-resolution image-taking operation mode in which an image is taken with a normal resolution and also having a high-resolution image-taking operation mode in which an image is taken with a higher resolution than the normal resolution, the image pickup apparatus being characterized in that in the focusing operation, the amount of driving the part of the imaging system from a current position for extracting a focus evaluation value to a position for extracting a next focus evaluation value is set to a value which differs between the normal-resolution image-taking operation mode and the high-resolution image-taking operation mode.
According to a further aspect of the invention, there is provided an image pickup apparatus including an imaging system including an imaging lens and an image-sensing device for converting an optical image formed via the imaging lens into an electric signal, the imaging system being adapted to perform focusing operation on the basis of the change in the focus evaluation value which occurs when a part of the imaging system is driven wherein the focus evaluation value is given by the amount of high-frequency components which are contained in a video signal output by the image-sensing device after passing through a frequency-pass filter, the image pickup apparatus having a normal-resolution image-taking operation mode in which an image is taken with a normal resolution and also having a high-resolution image-taking operation mode in which an image is taken with a higher resolution than the normal resolution, the image pickup apparatus being characterized in that the frequency-pass filter is set to have a frequency characteristic which differs between the normal-resolution image-taking operation mode and the high-resolution image-taking operation mode.
Other purposes and features of the invention will become apparent from the following detailed description and the accompanying drawings.